Porous ceramic materials are used as filter media, for ion exchange resins, as membranes, as absorbents as catalyst carriers, as a refractory brick material, as a heat insulator material, as a building material, etc., because of its excellent anticorrosion, heat resistance, and strength properties.
Various methods to produce such porous ceramic materials are known. Of these known methods, a method of producing a porous ceramic mold by baking a mixture of a ceramic material and a polyurethane foam to remove the polyurethane and at the same time to sinter the ceramic material has recently received increasing attention because it facilitates obtaining a molded article having a desired form.
For example, in accordance with a method described in Japanese Patent Publication No. 12927/79, 75 to 150 parts by weight of water is added to 100 parts by weight of a refractory material powder to prepare a slurry, a hydrophilic urethane prepolymer having an isocyanate group(s) at the terminal portion thereof, which is prepared by reacting a polyethylene oxide compound or copolymer consisting of from 95 to 70 of ethylene oxide and from 5 to 30 of alkylene oxide containing at least 3 carbon atoms, having an average molecular weight of from 1,000 to 20,000, with an isocyanate compound having at least 2 isocyanate groups at the terminal portions thereof, is added to the above-prepared slurry in an amount of from 3 to 30 parts by weight per 100 parts by weight of the water (i.e., weight ratio of the prepolymer/water is from 3/100 to 30/100), and the resulting mixture is then placed in a mold, dried and fired to produce a refractory and heat-insulative brick.
This known method, however, has the following disadvantages: (1) since the composition consisting of the refractory material powder, water and hydrophilic urethane prepolymer has a high water content, when a urethane foamed article molded in the mold is removed therefrom and then dried, the shrinkage of the urethane foamed article is great and cracking often occurs in firing the urethane foamed article; (2) since the composition has a high water content, the refractory material powder is easily precipitated and, thus, molding thereof is difficult; (3) since the urethane prepolymer content of the composition is small, the urethane foamed article obtained has poor strength, the time (cycle time) required until the urethane foamed article can be removed from the mold is relatively long, and the porosity of the article is small because of its low expansion ratio (expanded volume/original volume), and it is thus difficult to obtain an article having a small specific gravity; and (4) in order to increase the heat insulative properties, it is desirable to increase the closed cell content since the prior method is intended to provide a heat-insulative brick (i.e., the material produced according to Japanese Patent Publication No. 12927/79 teaches only providing a material having a high closed cell content).
A porous ceramic mold obtained by the foregoing known method is not suitable for use as a filter medium, a catalyst carrier, a sound-absorbing material, etc., for which the open cell structure is required.